Regarding to the more complicated ULSI devices and the multiple metallizations, it is no longer suitable to isolate metal wires with the most frequently used dielectric material, i.e., silicon dioxide (SiO.sub.2) which has a dielectric constant around 4.1-4.5, since capacitance between two adjacent wires is increased as the distance therebetween is decreased. As shown in the following equation: EQU C=ke.sub.0 A/d,
where k is dielectric constant, e.sub.0 is vacuum dielectric constant, A. is area and d is distance between two conductors. When the dielectric is maintained constant, the capacitance is increased as the distance d is decreased, which will increase the RC delay time, where R is metal wire resistance and C is capacitance, such that the transmission speed between devices is attenuated. Therefore, to cope with the problem derived from scale down of a chip circuit, it is desired low dielectric constant (k&lt;4.1) materials to control capacitances. The low dielectric constant materials are used for electric isolation between conductors, crosstalk depression and coupling capacitance reduction, thereby shortening RC delay time, enhancing transmission speed and improving coupling noise. There are some organic and inorganic materials to reduce the dielectric constant. Of course, the ideal minimum dielectric constant is 1.0, which is the dielectric constant of air; it is therefore to improve the dielectric constant toward 1.0 in the existing researches. In view of chemical attributes of materials, they can be classified into inorganic low dielectrics and organic low dielectrics. Chemical vapor deposition (CVD) is generally used for the growth of the inorganic materials, such as silicon dioxide doped with fluorine, carbon or hydrogen. Since the manufacturing equipment therefore is similar to the existing production equipment, they have been developed rapidly. The material of a more lower dielectric constant, such as lower than 2, can be obtained with a porous structure formed by either evaporation of a filming solvent or dissolution of a dopant in a film. Porous low dielectrics are highly expected.
U.S. Pat. Nos. 5,470,802, 5,494,858, 5,103,288, 5,548,159, 5,561,318, 5,569,058, 5,661,344, 5,747,880, 5,750,415 and 5,804,508 disclose similar processes for manufacturing porous materials. Namely, pores are formed under the control of volatility of various solvents in a film. In U.S. Pat. No. 5,494,858, for example, a mixture of tetraethylorthosilicate (TEOS), C.sub.2 H.sub.5 OH, H.sub.2 O, HCl and NH.sub.4 OH is prepared to form a gel film onto a wafer by spin-on coating. Gelation and aging may be accomplished by letting the wafer applied with a gel film sit in a saturated ethanol atmosphere for 24 hours at 37.degree. C. After aging is completed, pressure variation is utilized to remove the solvents in the film to form a porous film. The operational pressure can be either smaller than 1 atm to convert liquid state of the solvents into vapor state and thus to leave the porous film or a high pressure for the solvents to approach supercritical state or to be under supercritical conditions to compress the solvents into gases and thus to be moved. Care should be taken in these steps in order to avoid over shrinkage effect to the films. The porosity, diameter and distribution of the pores on the films depend on the reaction condition and the solvent kind. When the porosity is 80%, the dielectric constant is approximately smaller than 1.5. Since the surface adhesion and the heat resist performances are worse as the porosity is larger, a porous film is coated at either side with a layer of adhesive, such as SiO.sub.2, to improve adhesion and prevents material in the pores from absorbing water or atom migration.
In addition to the method in which solvent evaporation is used for manufacturing a porous film, a method of forming pores by removing a dopant in a film is known. Such as in U.S. Pat. No. 5,744,399, C.sub.60 or C.sub.70 fullerene is doped into a SiO.sub.2 film during the SiO.sub.2 film is formed, and then C.sub.60 is dissolved by a solvent or oxidized by O.sub.2 and/or O.sub.3 plasma to form pores.
In U.S. Pat. No. 5,785,787, materials such as anthroquinone and low melting temperature waxes, which are UV light degradable or sublimable, are used as dopants of a polytetrafluoroethylene (PTFE) film. The resultant PTFE film with 70% porosity reduces the dielectric constant from 1.9 to 1.2. Naturally, both sides of such a porous structure need adhesive layers; otherwise, it would peel off easily.
The methods of manufacturing a low dielectric constant porous film suitable to semiconductor processes can be categorized into two types such as shown above, i.e., solvent evaporation and removal of a dopant in a film. The former causes problems of shrinkage effect and poor adhesion to a substrate, while the latter causes problems of infiltration of residual dopant in the film at the subsequent processes. In view of these problems, herein, a non-solvent method of producing a low dielectric constant porous film is proposed.